Large-area Transition Metal Dichalcogenides Research Articles

Strong exciton-photon coupling in large area MoSe2 and WSe2 heterostructures fabricated from two-dimensional materials grown by chemical vapor deposition

Two-dimensional semiconducting transition metal dichalcogenides embedded in optical microcavities in the strong exciton-photon coupling regime may lead to promising applications in spin and valley addressable polaritonic logic gates and circuits. One significant obstacle for their realization is the inherent lack of scalability associated with the mechanical exfoliation commonly used for fabrication of two-dimensional materials and their heterostructures. Chemical vapor deposition offers an alternative scalable fabrication method for both monolayer semiconductors and other two-dimensional materials, such as hexagonal boron nitride. Observation of the strong light-matter coupling in chemical vapor grown transition metal dichalcogenides has been demonstrated so far in a handful of experiments with monolayer molybdenum disulfide and tungsten disulfide. Here we instead demonstrate the strong exciton-photon coupling in microcavities composed of large area transition metal dichalcogenide/hexagonal boron nitride heterostructures made from chemical vapor deposition grown molybdenum diselenide and tungsten diselenide encapsulated on one or both sides in continuous few-layer boron nitride films also grown by chemical vapor deposition. These transition metal dichalcogenide/hexagonal boron nitride heterostructures show high optical quality comparable with mechanically exfoliated samples, allowing operation in the strong coupling regime in a wide range of temperatures down to 4 Kelvin in tunable and monolithic microcavities, and demonstrating the possibility to successfully develop large area transition metal dichalcogenide based polariton devices.

Synthesis and characterization of WS2/graphene/SiC van der Waals heterostructures via WO3\u2212x thin film sulfurization

Van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs) and graphene have attracted keen scientific interest due to the complementary properties of the materials, which have wide reaching technological applications. Direct growth of uniform, large area TMDs on graphene substrates by chemical vapor deposition (CVD) is limited by slow lateral growth rates, which result in a tendency for non-uniform multilayer growth. In this work, monolayer and few-layer WS2 was grown on epitaxial graphene on SiC by sulfurization of WO3−x thin films deposited directly onto the substrate. Using this method, WS2 growth was achieved at temperatures as low as 700 °C – significantly less than the temperature required for conventional CVD. Achieving long-range uniformity remains a challenge, but this process could provide a route to synthesize a broad range of TMD/graphene van der Waals heterostructures with novel properties and functionality not accessible by conventional CVD growth.

Nucleation dynamics of single crystal WS2 from droplet precursors uncovered by in-situ monitoring

Transition metal dichalcogenides (TMDs) attract intence attention due to its unique optoelectrical features. Recent progress in production stage of TMD enables us to synthesis uniform and large area TMD with mono layer thickness. Elucidation of growth mechanism is a challenge to improve the crystallinity of TMD, which is regargeded as a next crutial subject in the production stage. Here we report novel diffusion and nucleation dynamics during tungsten disulphide (WS2) growth. The diffusion length (Ld) of the precursors have been measured with unique nucleation control methods. It was revealed that the Ld reaches up to ~750 μm. This ultra-long diffusion can be attributed to precursor droplets observed during in-situ monitoring of WS2 growth. The integrated synthesis of >35,000 single crystals and monolayer WS2 was achieved at the wafer scale based on this model. Our findings are highly significant for both the fundamental study of droplet-mediated crystal growth and the industrial application of integrated single-crystal TMDs.

Structural and Electrical Investigation of MoS2 Thin Films Formed By Thermal Assisted Conversion of Mo Metal

Due to the performance and economic benefit obtained by scaling, future semiconductor electron devices for logic functions will progress toward ultra-thin-body channels and 2-dimensional (2D) high carrier mobility materials. The significance is that small devices can be made to yield higher performance and greater energy-efficiency. To put this trend in perspective, fin-field-effect-transistor[i],[ii] (FinFET) technology was a research topic 7-8 years ago, but is now at the heart of the microprocessor in high-end smartphones.[iii] While the positioning of TMDs in electronic products of the future is still unsure, perhaps they will be more suited to low power, it is still worthwhile to understand how a relatively undeveloped system can reach maturity in less than a decade. Graphene is semi-metallic, which makes it difficult to switch off electron devices such as FETs. Bandgap engineering is required to open a bandgap of graphene, which is not an easy undertaking. This has motivated the scientific community to search for alternate 2D layer materials with semiconducting properties and better tuneability. Many TMDs are natural semiconductors with thicknesses on the nanometre scale. TMD semiconductors are now emerging as potentially useful materials, where more research is needed, in order to explore their properties and potential applications. Large-area synthesis is of great demand for the preparation of high-performance transition-metal-dichalcogenides (TMD) devices, however reports of device operation on large-area TMDs are sparse. In this work we fabricate MoS2 devices based on Thermal Assisted Conversion (TAC) of metal layers, and characterise the thin-films with material analysis combined with electrical device parameter extraction. Specifically we perform parameter extraction for MoS2 thin-films to determine sheet resistance (Rsh), resistivity (ρ), and activation energy (EA) of on-state current flow. For undoped MoS2, ρ was determined to be 191 Ω.cm at 25 °C. EA of the on-state current found to be 0.18 eV, points to a current generation mechanism possibly linked to a deep level in MoS2.In Fig. 1 are representative images of the 10 nm sulphurised Mo, forming approximately 20 nm of MoS2. In (a) a wide view of MoS2 on a SiO2/Si substrate. On the right-hand side the Ti/Au metal contact is visible. The MoS2 layer is continuous across the surface. In (b) the layered structure of MoS2 is visible in the higher resolution image. In Fig. 2 are representative measured current versus voltage characteristic in the MoS2 circular Transfer Length Method structure as a function of contact spacing. The current is linear with respect to voltage and passes through the origin. The current increases with decreased contact spacing as expected. The inset shows an optical images of the circular TLM structure. : [i] J. –P. Colinge (Ed.), “FinFETs and other Multi-Gate Transistors” ISBN 978-0-387-71752-4, Springer (2008). [ii] N. Collaert (Ed.), “CMOS Nanoelectronics: Innovative Devices, Architectures, and Applications” ISBN 9789814364027, Pan Stanford (2012). [iii] J. Borland, ECS Transactions, 69 (10) 11-20 (2015). Figure 1

Direct synthesis of ultra-thin large area transition metal dichalcogenides and their heterostructures on stretchable polymer surfaces

Abstract

Structural and Electrical Investigation of MoS2 Thin Films Formed by Thermal Assisted Conversion of Mo Metal

Large-area synthesis is of great demand for the preparation of high-performance transition-metal-dichalcogenides (TMD) devices, however there are only limited reports to date of device operation on large-area TMDs. In this work we fabricate MoS2 devices based on Thermal Assisted Conversion (TAC) of metal layers, and characterize the thin-films with material analysis combined with electrical device parameter extraction. Specifically we report on temperature dependent parameter extraction for Ti/Au contacts to MoS2 thin-films to determine sheet resistance (Rsh), resistivity (ρ), and the activation energy (EA) of on-state current flow. For undoped MoS2, ρ was determined to be 191 Ω.cm at 25°C. The activation energy of the on-state current was found to be 0.18 eV, pointing to the presence of deep levels in MoS2.